The thalamic visual relay nuclei are sites at which visual information targeted to the cerebral cortex is sorted, modulated, and distributed. In the cat, the main thalamic relay complex consists of the lateral geniculate (LGN) and medial interlaminar (MIN) muclei. The LGN-MIN complex is divided into several layers, each of which receives a unique combination of retinal inputs and each of which has a unique pattern of projection to the several areas of visual cortex. The manner in which these layers control activity of cells in visual cortex is the subject of this proposal. I have found that microinjections of cobalt chloride can be used either to reversibly inactivate or to permanently destroy segments of individual LGN-MIN layers without interrupting fibers of passage. I propose to use reversible and permanent inactivations, alone and in combination, to determine the effects on visual cortex of removing one or several of its thalamic inputs. As a continuation of electrophysiological experiments already under way, response properties of single neurons in areas 17 and 18 of the visual cortex will be studied before, during, and after reversible cobalt blocks of one or two of the major subdivisions of the the LGN-MIN complex. In a parallel seriesof experiments, the 2-deoxyglucose autoradiographic technique will be used to visualize patterns of disruption in cerebral cortex caused by cobalt lesions confined to individual subdivisions. The goal of this work is to determine to what extent particular outputs of visual cortex are modulated and coordinated through individual thalamic subdivisions.